Not just Shark Pictures:
Elasmodiver contains photos of sharks, skates, rays, and chimaera's
from around the world. Elasmodiver began as a simple web based
shark
field guide
to help divers find the best places to encounter the different
species of sharks and rays that live in shallow water but it has
slowly evolved into a much larger project containing information on
all aspects of shark diving and shark photography.

There are now
more than 10,000 shark pictures and sections on shark
evolution, biology, and conservation. There is a large library of
reviewed shark books, a constantly updated shark taxonomy page, a
monster list of shark links, and deeper in the site there are
numerous articles and stories about shark encounters. Elasmodiver is
now so difficult to check for updates, that new information and
pictures are listed on an Elasmodiver Updates Page that can be
accessed here:

There are many misconceptions regarding the evolution of sharks
and rays. Contrary to popular belief they have not remained unchanged for 300
million years. However, many of the families we have today have been in
existence for perhaps the last 150 million years. Compared to our own paltry 3.5
million years this makes the elasmobranch lineage very ancient indeed. The fossil
record is sketchy at best when it comes to sharks. Cartilage is preserved very
poorly so the body structures of many early sharks are purely speculative.
Fortunately for the paleontologists, sharks discard their teeth
on a regular basis and these teeth which fossilize well are often enough to allow accurate
identification of individual groups and help place them correctly in the
evolutionary time line. One of the pitfalls of using fossilized teeth for
identification is that the teeth of sharks sometimes vary significantly
depending on which area of the jaw they have come from. In the past this has led
to paleontologists inventing multiple species of extinct sharks that were
actually the same animal. During a shark's lifetime it's dentition also may
change which only adds to the confusion. In recent years there has been a
revision of extinct species that paid more attention to the herero-dental nature
of modern species, and consequently the list of extinct species has been paired
down.

A WORLD BEFORE FISH...

How life first came about on earth is a burning question that
many scientists and theologists have dedicated their entire careers to solving.
Be it by chance or divine intervention, it is likely that super heated compounds
developed a complexity at which point they were able to grow by absorbing the
chemicals around them. Complex compounds are a far cry from even the most basic
fishes but at some point (around 3.5 billion years ago) simple organisms started evolving. These
most ancient forms of life on earth were called stromatolites and they may still
be seen today. Stromatolites are simple organisms that cluster together and form
mounds not unlike coral reefs but with a far less complicated structure.

1.4 billion years later the first cells with a nucleus were
ooching around looking for a way to take advantage of both the chemicals in
their environment and each other. As is the nature of evolution, a radiation
took place after this point that enabled these basic creatures to fill
every niche that was available to them. By about 600 million years ago,
multi-cellular organisms such as jellyfish, worms, sea-pens, and more obscure
soft bodied invertebrates had taken over the ocean deserts.

540 million years ago near the beginning of the Phanerozoic Eon
(which lasts to this day and is split into the Cambrian, Palaeozoic, Mesozoic,
and Cenozoic Periods) Most of the modern groups of animals were developing. This
era heralded the development of shelled creatures which have been preserved very
well in the fossil record.

There is some conjecture regarding how primitive fishes managed
to develop from these invertebrates. Most likely, fishes evolved from the larval
forms of their predecessors. Most invertebrates release eggs and sperm which
after combining, develop into free swimming, fish-like larvae. These larvae
drift within the planktonic soup until they reach a point of metamorphosis. In
many groups such as corals and tunicates this metamorphosis is highly dramatic,
involving a complete restructuring of the body to adapt to a sessile lifestyle.
However, if the planktonic environment were rich enough it would be prudent for
some larvae to remain as free swimming organisms permanently. Which group of
invertebrates are responsible for spawning the first fishes is not known but it
is possible that it may have been the echinoderms (seastars and sea cucumbers)
as they share an early cell developmental process with vertebrates. Some
scientists believe that sea squirts are responsible. Recent fossil evidence also
points to a now extinct animal called a conodont that was characteristically
fish-like and may have been the true predecessor.

THE AGNATHA

Around 500 to 450 million years ago the first primitive fishes
appeared in the fossil record. They were the Agnathans and they were
characterized by two (dorsal and ventral) bony shields on the head with many
trunk scales tapering towards a primitive caudal fin in which the notocord
turned upwards rather like the sharks of today. One notable difference was that
the first agnathans lacked paired fins (pectoral and pelvic fins).

Agnathans were very successful
and diversified to dominate every niche available until well into the Devonian
period towards the late Paleozoic. Eventually, some groups developed many of the
characteristics associated with vertebrates today including paired limbs,
cellular bone, a complex sensory line system, dentine, and complex eyes and
inner ears. Sadly these pioneers of all that represents our ancenstry, mostly
vanished during the Devonian extinctions. Today the two remaining groups of
agnatha are
represented by the Hagfishes and Lampreys. They are jawless, limbless, and have
rudimentary, cartilaginous back bones.

THE RISE OF JAWED FISHES

At some point during the Ordovician Period also known as the early
Paleozoic, the major groups separated from eachother. Between about 400 and 350
million years ago the seas were beginning to fill. The major groups were:

Osteichthyes - the bony fishes which eventually
outnumbered all other vertebrates by species.

Crossopterygii - which split to form the
Porolepiformes which evolved into salamanders and the Osteolepiformes
which eventually evolved into frogs and toads, reptiles, birds, and mammals.

Dipnoans - the lungfishes

Acanthodians - also called the spiny sharks -
Superficially shark like with strong broad spines strengthening all of their
fins except the caudal fin which was upturned (heterocercal) as in the true
sharks. Their bodies were covered with small flat bony scales. The
Acanthodians survived into the early permian about 200 million years ago.

Placoderms - a large group of fishes covered with big bony plates and blade like jaws that proliferated during the Devonian
and then died out at the beginning of the carboniferous period.

Although Chondrichthyes are rooted in the Ordovician period, the first
well-preserved early shark fossil to be discovered was Cladoselache dating
from approximately 350 million years ago. The fossil of this shark was
found miraculously intact in the Cleveland Shale of Lake Erie. It was so well
preserved that its muscle fibers were visible as were its kidneys. Cladoselache
Had two low dorsal fins both with prominent spines, broad based pectoral fins
and eyes set far forward on the head. The mouth was at the front of the head as
opposed to the under slung mouths of modern sharks, and the teeth had a large
central pointed cusp with a smaller point on each side. Although cladoselache
was almost certainly not the first ever true elasmobranch, armed with Cladoselache,
Paleontologists were able to categorically state that by this time, elasmobranchs had arrived.

TWO VISIBLE SUBCLASSES

At the end of the Devonian another group of cartilaginous fish
became evident. These fish known as Chimaeras or Ghost sharks had distinct mobility
differences in their skeletal structure. The upper jaw was fused to the skull,
the pectoral fins were large and able to flap unlike the rigid fins of sharks
and the rear of their bodies tapered into thin whip-like tails. Due to the great distance
between the chimaeras and the modern elasmobranchs, extant chimaeras are placed in
the separate subclass (Holocephali)and presently consist of three
families and around 40 species.

AN EXPLOSION OF FORMS

During the evolution of chrondrichthyes there have been many groups
with bizarre appearances. Sometimes these families are collectively referred to
as "paraselachians" . Many fossil skeletons contain unusual
appendages. Most of which have as yet not been conclusively explained.

Some examples of these paraselachians include:

Stethacanthus - a Cladodont which lived
through the Silurian Period between 380 and 300 million years ago. It had a
modified first dorsal fin that terminated in a spine covered pad reminiscent
of an inverted scrubbing brush. Its forehead also had a similar surface.
These surfaces may have been used for pinning prey to the ground or possibly for mating.

Helicoprion - from the Permian Period, had a conveyor
belt of teeth that spiraled out of its lower jaw and a thin corresponding
line of sharp teeth in the upper jaw. The lower whorl of teeth rotated
out of the jaw as the shark grew. Unlike most sharks it retained the smaller
previous teeth which rotated back into the jaw forming a spiral or whorl not
unlike the growth pattern of a shell. The two dermal surfaces sliced against
each other giving it a formidable shearing weapon.

Falcatus - from the carboniferous period had a
curving, forward facing appendage in place of its first dorsal fin. It has
been suggested that only the male may have had this sword like structure.

Xenacanthus - a member of the pleurocanthids.
It had a long backward facing spike extending from the back of its skull and an
eel like or ribbon like fin running down the length of its back.

Iniopteryx - Iniopterygians lived from the Devonian into the
Carboniferous period. More closely related to modern day chimaeras, they had
flexible pectoral fins which were disproportionately long and rayed for
strength. It is unclear whether these "wings" were used to glide
above the water or to paddle under it. The leading edge of the wings were
covered with sharp toothy denticles.

Most of these evolutionary experiments were probably adaptations
to the demands of life in an ever more competitive environment. During the
Carboniferous Period the ranks of the sharks swelled to their greatest diversity
ever but towards the beginning of the Permian Period many ancient forms became
extinct along with the majority of the more experimental forms.

ENTER THE NEOSELACHIANS

As the Permian Period was drawing to a close the seas were
filling with Actinopterygians - the ray finned fishes. This was a food source
that could not be ignored by the ocean's predators. In response, the elasmobranchs began to radiate again and during the early Triassic a shark
appeared in the fossil record that was similar enough in appearance to modern
day sharks to be considered one of the first of the "modern sharks".
The name of this shark was Palaeospinax.

Palaeospinax was morphologically similar to the dogfish
of the family squalidae. It had a calcified sectioned vertebral column instead
of a continuous notochord, its two dorsal fins had supportive leading edge
spines, and most notably it had the under slung mouth of a modern shark.

NEOSELACHIAN RADIATION

Amongst the first of the presently extant sharks to swim in the
seas were the slow swimming Horn sharks and the Cow sharks but towards the mid
cretaceous the food supply in the mid oceans was enough to push the
development of fast moving predators that could pick off large, schooling, off
shore fishes. At the time the seas were ruled by enormous icthyosaurs and
plesiosaurs so this new food source did not come without risk to the
sharks.

During the Cretaceous, most of the present genera were firmly
established and then around 60 million years ago at the end of the Cretaceous a catastrophe
occurred which wiped out the dinosaurs and many other species, leaving the
remaining sharks as the supreme rulers of the oceans.

CARCHARODON MEGALODON

About 50 million years ago, a super-predator evolved, the size of
which the world had not previously seen. Megalodon was similar in shape and
dentition to the White shark (Carcharodon carcharias) the most notorious
shark of today's temperate oceans. Its size however was spectacular. The largest
of its fossilized teeth that have been found to date have measured over six
inches long from point to base. Extrapolating this information and using
the relative proportions of Carcharodon carcharias as a blue print, it would not be unreasonable to assume that Megalodon reached somewhere between 50 to 100 feet in length. Sadly, but perhaps
just as well for us, no Megalodons have been seen for some time. Estimates on the time of
its extinction vary widely. Some popular fiction writers would like us to believe
that Megalodon is still down there somewhere lurking in the shadows. More likely, Megalodon faded away some time within the last 30 million years due to a
combination of a waning food supply and a changing climate.

RAY RADIATION

Meanwhile, back at the end of the Triassic, at about the same
time as Palaeospinax was swimming around the coastline of the super continent
Pangaea, another group of sharks were adapting well to the bottom terrain of the
shallow slopes. By the upper Jurassic Period the first guitarfishes were
grubbing around for food and blending into the bottom sediments. These rays were
a little more primitive than those of today. The main differences being a more
shark like skeletal structure and the presence of fin spines. It has been
suggested that all modern rays were derived from primitive guitarfishes but it
is unclear exactly where the families are linked. The most recent
addition to the batoid tree are the stingrays which showed up a mere 60 million
years ago. So far so good; stingrays have adapted extremely well to the changing
state of our modern oceans. They fill the shallows of most tropical and temperate continental
waters.